Lead frames comprise metal plates in which fine patterns such as inner leads and outer leads are processed. After mounting a semiconductor chip on the lead frame, terminals of the semiconductor chip and the inner leads are electrically connected to each other.
Recently, there has been an even stronger demand for a higher packing density and higher integration of semiconductor chips. Corresponding to such a demand, lead frames for mounting semiconductor chips thereon have also been developed which are of fine shapes and highly accurate dimensions. In particular, great weight has been given to increasing the number of pins per lead frame. With an increase in the number of pins per lead frame, a processing technique is required which can process distal end portions of inner leads to be smaller in pitch and finer in shape.
Conventional techniques for forming such lead frames having a number of pins and a narrow pitch from metal plates have been primarily practiced by a process utilizing chemical corrosion, particularly wet etching using an etchant (hereinafter referred to simply as etching). By that conventional processing method, lead frames having satisfactorily fine configurations can be easily processed.
Meanwhile, there is also known a method of processing metal plates, particularly lead frames, in combination of etching and laser cutting as disclosed in JP-A-3-123063 and JP-A-4-37493. The processing method disclosed in the former JP-A-3-123063 comprises cutting the portions having a narrow pitch, such as inner leads, by irradiation of a laser beam, and processing the portions not having such a narrow pitch, such as outer leads, by punching or etching. The processing method disclosed in the latter JP-A-4-37493 comprises etching a part of the total amount which is to be processed in the direction of plate thickness to form a recess having a curved inner surface. Then a post-cutting operation is undertaken the remaining part, which has been left not penetrated by the etching, by irradiation of a laser beam so that the remaining part is thoroughly penetrated to form inner leads having a narrow pitch.
The laser cutting employed in the above methods is suitable for fine processing because a laser beam can be condensed into a very small diameter, enabling metal plates, particularly narrow-pitch inner leads of lead frames, to be processed easily and reliably.